A wide variety of different technologies have been developed in the last few years for heat sources with temperatures from 100° C. to 200° C. and these allow the heat thereof to be converted into mechanical or electrical energy very efficiently. It is primarily thermodynamic cycles with a working medium comprising at least two substances with non-isothermal evaporation and condensation, such as the Kalina cycle, which are distinguished by particularly high levels of efficiency. Kalina cycles, as are known for example from EP 0652 368 B1, use a mixture of ammonia and water as the working medium, with the non-isothermal boiling and condensation process of the mixture being exploited to increase the efficiency of the cycle, for example compared with the conventional Rankine cycle.
Since the working medium used can decompose above a certain temperature however (hereinafter called the “decomposition temperature”) use of cycles of this type with heat sources with temperatures above the decomposition temperature turns out to be difficult. In the case of the Kalina cycle with an ammonia-water mixture as the working medium, the ammonia-water mixture begins to decompose above 250° C., i.e. there is chemical break up of the NH3 compound (2 NH3→N2+3H2), so hydrogen and nitrogen are liberated. For this reason the cycle does not operate above temperatures of 400° C.
On the other hand, however, use of such cycles is also attractive in specific cases for heat sources with temperatures of above 400° C. This applies for example to gas turbine plants, constructed in the past, without steam generation and to combined gas and steam turbine plant. As a result of enormous cost pressures, in particular the operators of old plant with low efficiency levels are forced to improve the economic efficiency of their plant.